Method of making an article of shellac bonded abrasive material



iatented Jul 3, 1934 METHOD OF MAKING AN ARTICLE OF SHELLAC BONDED ABRASIVE MATERIAL Duane E. Webster, Worcester, Mass., assignor to Norton Company, Worcester, Mass a corporation of Massachusetts No Drawing. Application November 24, 1930,

' Serial No. 497,845

3 Claims.

This invention relates to abrasive articles and more particularly to a method of making articles of abrasive grains bonded by means of shellac,

" which are used as grinding wheels, abrasive stones, and the like.

It has been customary to make a shellac bonded grinding wheel by forming a dry granular mixture of-shellac and abrasive grains, placing the mixture in a mold and heating the mass to melt the bond and render it plastic, subjecting the mass while the shellac is hot and plastic to a definite hydraulic pressure to compress it into the desired shape and thereafter heat setting the bond to form an integral body. The process has also involved the expensive preliminary operations of mixing the shellac and the granular abrasive material in a heated kneading machine to incorporate the bond thoroughly with the abrasive and of granulating the mass when cool to obtain the 2G material in a satisfactory and usable form.

Inorder to prepare the ingredients for molding purposes, it has been customary to partially prepare and store large quantities of the expen-= sive ingredients in amounts suflicient to last over a production period of several weeks, and to effect this treatment by means of special machinery suitable for handling the material in a heated condition. Small sized machines have not been used efiiciently, due to the difficulty involved in properly controlling the proportions of the ingredients of a small mix and of regulating the temperature and operating conditions of the mix; yet orders may call for but a small quantity of wheels at a time. On the other hand, it has been 35 found difficult to mix or crush small quantities of the shellac abrasive material in the large machines, particularly since the dimensions of the apparatus are such that the small amount of material present is easily thrown about and readily 4o sifts between the operating elements and is not acted upon. Moreover, such methods are not economical since the labor expense and the equipment involved are the same for the small mixings as for the large. The prior process is also disadvantageous in that it is impossible to heatset the product immediately after the pressing operation, but this must be delayed until the material has cooled sufliciently to permit removal from the mold. This results in a considerable expense in both capital investment and time lost in press output, as well as the tie-up of a large number of duplicate molds of each size during the period of cooling.

Due to the many difiiculties involved in producing such an article from the hot sticky mass, the

' easily and economically carried out and which the wheels have often been found with closed pores or portions of difierent density, resulting in hard and soft spots in the wheel structure. In practice, it was not feasible to control the pore volume and the abrasive content in the wheel, hence such prior methods have not resulted in the production of wheels of controlled volume structure.

It has also been found that such prior methods often produced a swelling in the product, owing to the presence of volatile matter which, after being driven off from the bond during heating, was confined within the closed pores of the mass, thereby rendering thewheel practically useless for grinding purposes. 70

It is accordingly the primary object of my invention to overcome these disadvantages and to provide a method of making an abrasive article of shellac bonded abrasive grains which may be is particularly applicable to the production of articles ofgreater porosity than those heretofore produced by standard methods.

A further object is to provide a method of making shellac bonded abrasive articles which are of controlled volume structure, and in which the volume percentage of abrasive, bond and pores may be of definite predetermined values.

It is another object of the present invention to eliminate the preliminary preparation and storage of partially processed material and to devise an economical method of making such shellac bonded abrasive articles whereby the raw ingredients may be utilized in either large or small amounts as required to produce the desired sizes and number of abrasive articles.

Further objects will be apparent in the following disclosure.

In accordance with this invention, I propose to make a mixture of abrasive grains, such as crystalline alumina, silicon carbide or other suitable abrasive, and shellac bond in the desired proportions, then to render the mixture slightly plastic, as by means'of a plasticizing medium, and to press these ingredients while in a cold state to the desired shape and size, and thereafter heat-set the pressed mass. By utilizing predetermined proportions of abrasive and shellac and then compressing a weighed amount of this predetermined mixture to a definite volume, I can produce a porous bonded abrasive article in which the percentages by volume of abrasive grain, bond and pores are predetermined and exactly controlled. If the volume percentage of abrasive grain remains constant, then a series of wheels m V of progressively increasing grades of hardness 1 the structure of the article may be definitely controlled by the simple expedient of mixing precalculated proportions of the ingredients and then placing a weighed amount of the mixture in a mold of known size and compressing the mixture to a definite volume which has been calculated to give a required weight per unit of volume.

In order that the abrasive grains and shellac may be compressed to a desired structure, it is desirable that the bond have a low plasticity during the initial molding operation and that it not be permitted to flow when compressed to such an extent as to close the pores of the wheel. To this end, I prefer to employ a solvent for shellac capable of causing some of the shellac to stick to the grains and in turn adhering to dry particles of shellac and thus coating the grains with a semi-plastic mass, so that the bond when compressed will stick together and form a wheel structure that may be readily handled outside of the mold.

As an illustration of this invention, I may form a grinding wheel of shellac bonded grains of abrasive material by the following procedure. The granular abrasive material may be initially wet with a plasticizing solvent capable of dissolving shellac without chemically reacting therewith and'causing it to adhere to the grains and of holding a layer of dry shellac as a coating uniformly distributed over the grain surface. The solvent should be used in an amount sumcient to make the coating slightly sticky, so that the grains will stick together when cold pressed, whereby the article may be removed from the mold before the heat-setting operation.

The solvent utilized in the preliminary mixing operation may be one of various materials, such as ethyl lactate, furfural, denatured alcohol or other shellac solvents adapted-to coat the grain and cause the shellac to adhere thereto as a dissolved coating of fusible unreacted shellac which is uniforml'y distributed around the abrasive. Dry shellac particles may, in turn, adhere to the sticky coating on the grain, whereby it may be readily molded in a cold condition. The amount of solvent used depends upon the kind and properties of the solvent, the size of the abrasive and the proportions of shellac and abrasive in the mixture. Ordinarily the amount of solvent added is in the proportions of from 3 to 17% by weight of the bond. The shellac binder is then added and the materials thoroughly mixed, the proportions of the bond varying in amounts from 5 to 20% by weight of the mixture.

Having thus obtained a mixture of abrasive grains and shellac in the desired proportions as has been precalculated for the required structure of the abrasive article, the semiplastic mass is placed in a mold of required dimensions and shape and compressed to the desired volume, while in a cold condition or at a temperature at which the shellac will not flow readily underthe compressional force. This insures that the article will remain porous. The pressure applied to this cold mass is regulated to bring the article to the required thickness so as to give a definite weight per unit of volume and a desired structure. Thereafter the article is removed from the mold and heated under suitable temperature conditions to set the shellac and form an integral body.

As a specific example of a composition suitable for a grinding wheel or similar abrasive article made in accordance with this invention, the following ingredients and approximate proportions may be employed:

Granular abrasive material of suitable grit size (crystalline alumina or silicon carbide) 9 lbs. Powdered shellac 1 lb. Ethyl-lactate 50 cc.

These ingredients are so proportioned as to constitute calculated volume percentages of abrasive, shellac and pores in the finished article.

The abrasive grains selected, which may be of any desired size, are mixed with the solvent in a suitable mechanical mixing machine. The solvent may be introduced to the grains in any suitable manner, but it is preferred to add the liquid solvent slowly by pouring or spraying it over the grains while the mass is being mixed and to continue the mixing until the fluid is distributed throughout a considerable portion of the mixture so as to wet all of the surface of substantially every abrasive particle. Thereafter a definite amount of shellac in the form of powdered material is added to the wet mass and the mixing continued until the shellac is thoroughly incorporated throughout the abrasive mass. These ingredients should be mixed rapidly to prevent the formation of lumps.

This mixture may now be formed into a shaped article by placing a definite weighed amount of the material in a mold of exact dimensions and then compressing the mixture to a calculated thickness so as to form an article having a predetermined weight per unit of volume. The mold may be an open topped container having a cover slidably fitting within its side walls, so that when placed between the movable plates of a press, the mass therein may be compressed under a suitable pressure while in a cold state until the article has a calculated thickness. That is, it is compacted into an article of the desired shape and size without reference to the degree of pressure applied, the only measurements made being the weights and proportions of the materials used and the total volume of the article. Due allowance will be made in the calculations to take care of any change in volume during the heat-setting operation.

When the article is removed from the mold, the latter is ready for a subsequent filling and pressing operation without requiring any heating or cooling. The pressed article thus shaped is'subjected after removal from the mold to suiiicient heat to set the bond, as by baking in an oven for several hours at a temperature of from 300 to 350 1''. until'the shellac has become converted to a hard, solid condition and forms an integral bonded structure. The article may then be removed from the oven and allowed to cool down to room temperature in order to toughen the structure.

It is obvious that various changes in the kinds of ingredients and their proportions may be made within the knowledge of those skilled in the art of making bonded articles of this type.

By using a plasticizing agent for the shellac as above described, the bond may be brought to a sufliciently plastic condition to enable it to stick to the grains, and permit the grains to be mixed easily and thoroughly with the shellac binder and then shaped while cold into the desired form.

or flow together and close the pores between the grains it will insure the production of a coherent porous body having an open structure and a predetermined porosity. The volume percentage of the pores is determined by the percentages of bond and abrasive. If, for example, one uses 50% by volume of abrasive and 20% by volume of bond, there is necessarily 30% of pore space between the ingredients of the wheel. Furthermore, abrasive wheels may be made either dense or porous, whether the grains are coarse or of small size, and the grade or hardness may be regulated and controlled more accurately within desirable limits depending principally upon the proper control of the volume percentages of abrasive, shellac and pores in relation to the total eration. Since no heating is required for makingthe mixture and for molding the article, the process is much more economical than heretofore. Also a greater factory production for each mold may now be obtained, and therefore the cost of mold equipment is materially reduced. This process is particularly desirable in that it makes it possible to control the structure of a grinding wheel and to make it feasible to provide a series of wheels having the same volume percentage of abrasive grain which differ uniformly in their grades of hardness and grinding characteristics.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. The method of making an article of shellac bonded granular abrasive material comprising the steps of mixing the abrasive grains with shellac in a quantity proportioned to form a porous bonded article of the desired grade of hardness, rendering the mass slightly plastic in the cold state by means of a solvent and plasticizing material which dissolves the shellac without chemically reacting therewith and thereby forming a stable mass of bond adhering to the grains, shaping the abrasive mixture in a mold and compressing the mass into a coherent porous body without heat-softening the shellac, and thereafter heat-setting the shellac to bond the grains and form said article.

2. The method of making an article of shellac bonded abrasive material comprising the steps of mixing definite amounts of abrasive grains and shellac powder with a small amount of a solvent for the shellac which is capable of causing a coating of shellac to be distributed over the surface of each abrasive grain, but without detrimentally reacting with said bond, thereby forming a stable, moldable mixture of abrasive grains and bond therefor, which may be molded to a structure of desired pore volume, and thereafter molding the mixture, while in a cold condition, and then subjecting the molded material to heat and causing the shellac to be set and thus unite the abrasive grains into an integral structure.

3. The method of making an article of shellac bonded abrasive material comprising the steps of wetting the grains with a slight amount of a neutral liquid capable of. dissolving shellac, without detrimentaily reacting therewith, and then mixing with the wetted grains a definite proportipn of shellac powder and causing the same to be distributed uniformly among the grains and to adhere thereto, thereby forming a stable and moldable mixture of bond and abrasive, and thereafter molding a predetermined amount of the mixture, while in a cold condition, and pressing the sameto cause the material to occupy a predetermined volume and thereafter heating the pressed article to a temperature at which the shellac is matured and set it to a hard condition and thus unite the abrasive grains into an integral body of desired porosity.

DUANE E. WEBSTER. 

